Synthetic oil is better than regular oil as a lubricant. It resists thermal break down better, and has the unique ability to cause an engine to run cleaner over time.
That is a general rule.
Depending on your engine milage you should switch over. If the engine has just been rebuilt dont use synthetic untill about 4 or five oil changes AFTER the break in period.
Over 200 000km dont switch to synthetic.
I can explain why if you like but thats just a general rule.
I live in Canada, i work at a shell station, i can tell you from personal experiiance that synthetic 10 30 flows better at -30C than regular 5-30...
Stay away from the "synthetic pretenders" such as valvoline, castrol, and pennzoil. Use a synth that's know to be good such as Amsoil or Mobil 1. I've used Mobil 1 full synth 10-30 since I had around 110k miles, and I have 198 now. My motor runs very well and I only use 1.5 quarts or so between changes, and I think most of that is going out my bad turbo seals. I also use Mobil 1 oil filters.
WHATEVER YOU DO STAY THE HELL AWAY FROM FRAM!!!!
Mobil 1 is very good. The only problem with putting synth in a high mileage engine is that it causes oil leaks and that your car may burn more synthetic than it does normally. Still it's good for the engine.
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